Effect of internal stress on the hydriding kinetics of nanocrystalline Pd thin films

Internal stress plays a key role in the design of materials that interact with hydrogen, for instance for sensing or storage applications. While increasing evidence in the literature points to an effect of internal stress on the equilibrium H-uptake, it is still an open question to what extent the hydriding kinetics can be affected by stress. Here, the hydriding kinetics of nanocrystalline Pd thin films is studied with varying degrees of growth-induced internal stress levels. The experimental technique consists of a high-resolution curvature measurement set-up. After mounting the sample inside a high-vacuum chamber, a H-containing gas mixture is introduced to generate instantaneously a given hydrogen partial pressure (pH2) inside the chamber. The resulting interaction of H with the Pd layer then leads to an expansion of the thin-film system constrained by the underlying substrate, inducing in turn a change in the sample curvature. The curvature evolution, obtained in situ at different pH2, is first interpreted with a self-consistent kinetic model in order to identify the different rate-limiting steps during a hydriding cycle. Next, the corresponding rate constants are calculated, as well as their variation with internal stress. The internal stress does not affect the rate constant of the adsorption-limited regime in the range 60-450 MPa, leading to an average value of 1.1 ± 0.2 × 1021 mbar−1 s−1 over the experimentally covered pH2 range, 1-10 mbar. As to the absorption-limited regime, a significant stress effect was recorded on the absorption rate constant of the very initial stages of hydriding, leading to an activation volume of 17.6 ± 2.3 Å3 for H-absorption into our Pd thin films. This value, combined with an initial internal tensile stress of the order of 450 MPa, leads to a decrease of the activation energy for H-absorption of 3 kJ mol−1. This effect induces up to a factor 3 increase in the room-temperature absorption rate in the initial stages of hydriding.